Gas tube arresters are commonly used to protect telecommunication lines from electrical surges. Since gas tube arresters need to be hermetically sealed to perform the protection function, there is the possibility that the gas will vent from the arrester resulting in a much higher breakdown voltage than originally intended and rendering the gas tube unable to protect. To provide for continued protection should venting occur, arresters are provided with back up protection in the form of an air gap or with a solid state device, for example, a metal oxide varistor (MOV).
U.S. Pat. No. 5,388,023 discloses a gas tube protector with one or two MOVs used as a back up. A gas tube protector with a back up device is sometimes referred to as "vent safe." In such protectors, the gas tube is sometimes termed the "primary protector." Gas tubes are widely used as primary protectors because of their ability to repeatedly divert large surge currents to ground and remain functional to protect.
Because it is desired that the gas tube and not the back up divert surges to ground, the operate voltage of the MOVs are set higher than the operate voltage of the gas tube. The '023 patent discloses "5 to 10% or else between 10 and 40% above the response voltage of the overvoltage arrestor." With the response voltage of the MOVs set in such a range, the MOVs are intended to only divert surges if the gas tube has vented. In normal operation, the gas tube alone is intended to divert surges to ground. The '023 patent defines the response voltage of the MOV as the voltage at which the varistor conducts a current of 1 mA.
U.S. Pat. No. 5,500,782 also discloses the use of a MOV with a gas tube with the clamping voltage of the MOV above the breakdown voltage of the gas tube. While the '782 patent uses the term "hybrid" to describe the disclosed protector arrangement, the MOVs are used as a back up protection device in the event that the gas tube should vent. The '782 patent teaches that the 1 mA clamping voltage of the MOV is selected to be just above the upper tolerance of the DC breakdown voltage of the gas tube so that the gas tube acts as the primary surge protector and the MOV provides back up protection in case the gas discharge tube fails to operate properly.
MOVs are preferred over traditional air gaps because they have a more repeatable clamping voltage than air gaps in response to fast rising voltage transients and they are not susceptible to contamination and moisture like the air gap.
One drawback of gas tubes as protectors is their ionization time which contributes to a higher peak surge voltage, or impulse breakdown voltage. The DC breakdown voltage of a gas tube is the voltage at which a gas tube will ionize when the voltage is increased slowly, for example, 100 volts per second. By raising the voltage slow enough such that the ionization time of the gas tube is taken into account, the DC breakdown voltage of the gas tube can be determined. If the voltage is a surge voltage, for example, 100 volts per microsecond, the gas tube will breakdown at a voltage predominantly higher than its DC breakdown voltage because of the ionization time of the gas tube. This higher voltage is termed "surge breakdown voltage" or "impulse breakdown voltage." It is possible that the impulse breakdown voltages of the gas tubes are sufficiently high that there could still be a shock to a person that is in contact with the circuit at the time of the surge Therefore, it is possible to have personnel injury and/or equipment damage from a gas tube protected circuit.
Therefore a need exists for a telecommunications protector with a robust gas tube protector as the primary protector but that is "assisted" by a secondary protector against fast surges to lower the impulse voltage. A further need exists for a protector where the secondary protector is capable of acting as a back up should the gas tube vent.
Another drawback of gas tubes is that there are wide variances of the DC breakdown voltages among gas tubes of the same type and made by the same manufacturing process. This variance is much wider than the variances for other components such as MOVs and fusible elements. Thus a need exists for a gas tube protector with a secondary protector that lowers the impulse voltage that takes into account the wide range of DC breakdown voltages across a population of gas tubes of the same type.
While both the '023 and '782 patents disclose incorporation of "fail safe" arrangements in the protector to short to ground any surges that overheat the protector, one drawback of the '782 patent arrangement is its bulkiness. The MOVs are spaced from the gas tube and arranged in a manner that takes up more space than the arrangement in the '023 patent which compactly locates two MOVs on opposite ends of the gas tube while still incorporating a thermal overload short to ground arrangement. Either one of the MOVs alone or the gas tube alone if overheated will melt the thermal element in the '023 arrangement to short to ground. Also, the MOVs in the '782 patent are not of sufficient size to impact the surge voltage under normal operating conditions.